REST api v3: Create Pull Request not working anymore?

I was working on creating some kind of automation on Github with merging PRs (pretty much just trying to trigger a new Pull Request between staging and testing server each time staging is deployed). 

The API in general is working fine with my access key etc, I can view open PRs in the same org_name and repo_name that I am using for POST/Create PR with no problems. But when I try to create a new pull request I keep getting this error: 

“message”: “Invalid request. For ‘links/0/schema’, nil is not an object.”

and request I am trying with CURL or Postman is (POST ofc) https://api.github.com/repos/org_name/repo_name/pulls?access_token=token_that_worked_for_get_private_info&title=test&head=master&base=development and this matches docs for https://developer.github.com/v3/pulls/#create-a-pull-request in my opinion. 

Has the API changed without the documentation being noticed and this feature is now only supported by the v4 GraphQL API or am I just doing something wrong here?

edit: used hub to do this eventually.

@exhibiton , I had a similar problem with python requests module and when I used the json keyword over params keyword, it worked. So, the problem is the data you are posting as the status code indicates.